To clear cache go to xbox dashboard, go to my Xbox (Says play disc/displays avatar) then go to system settings (think that's what its called) all the way to the right, go to memory, clear cache (I think you press Y when at the memory part).
To start a new mission go to replay mission and select the one you want/difficulty if the above fix doesn't work
Just post if it works/fails epicly (I'll read it when I get back on)
